What Was Changed?To those not familiar with older editions, it used to be that when a character was struck by some sort of non-weapon attack (like a fireball being flung at them or being exposed to poison gas) then they were told to make a "saving throw" versus the type of attack. Depending on the edition, this took different forms -- older D&D used a basic roll, and characters had specific bonuses given the circumstances -- for example, being more resistant to poison would mean you got a bonus for "saves vs poison". In 3rd edition the throws were unified into three general categories -- Fortitude (physical resistance), Reflex (reaction and quickness), and Will (mental or spiritual toughness). Instead of a "Save vs Poison" a player would roll a "Fortitude Save" -- rolling 1d20 + their Fortitude Save Bonus. This was great because it simplified the number of potential modifiers the characters needed to keep track of, allowed the classes to advance at different rates (so fighters were more resistant to physical effects, but your rogue had a much better chance of dodging that fireball). Often the Save DC was a slightly complicated calculation based on the attackers Caster Level and relevant ability score.
With 4th Edition, WotC changed the system again. First, they moved the die roll from the defender to the attacker's side -- now instead of being hit by an effect and rolling the appropriate save, the attacker just rolls to hit your non-AC defense directly. If an attack had secondary effects, the defender would roll a "4e Save" -- roll 1d20, with you saving on a 10-20 and failing on a 1-9. Secondly, they essentially eliminated the varying scales at which different classes gained NADs in favor of mirroring the progressive gain over levels that you get with 4e Armor Class (10 + 1/2 Level + Ability +Misc Bonuses). They even go a little further and group the attributes, so instead of CON/DEX/WIS determining your Fortitude/Reflex/Will, it's the largest of [STR/CON], [DEX/INT]. and [WIS/CHA]. This tends to flatten out characters a bit more (since now your wizard might be just as dodgy as your rogue and your wisecracking Bard might be able to resist vile suggestion as well as your stalwart Cleric -- better perhaps than your Paladin!); however, I don't know that that flattening is too much of a problem, because to some extent this happened indirectly in 3e because the class-based bonuses tended to be such a big factor. Besides, a lot of the more "Charismatic" people I know tend to be pretty pig-headed, so maybe it makes sense for them to have a high willpower.
What Was Gained?
First and foremost, you can definitely appreciate the aesthetic impact of having all of a character's attacks use the same model of "Roll to hit, roll for damage" regardless of whether they are basic AC attacks or more sophisticated attacks versus other defenses. I would be pretty darn hesitant for a player new to D&D to sit down and play a 3e Wizard, but with 4e it certainly seems a lot more accessible. This is doubly true when you consider the complexity of determining the save DCs for various effects. It could be simplified and automated even under the 3e model, but you have to appreciate at least somewhat the convenience of having them up front and easily derived.
This change also eliminates what I would consider one of the most well-intentioned but poorly-executed aspects of 3rd edition -- Touch AC. In 3e, some attacks (like a wizard flinging a lightening bolt) still required an attack roll to connect, but wouldn't be blocked by armor. Basically, the character could "dodge" the attack but not block it. Unfortunately, this had to be different from Reflex saves because to-hit, AC, and Save bonuses scaled differently. This involved players basically recalculating their AC, but only counting some sources. It was a pain and, while neat from a simulationist perspective, always tended to grind play to a halt while players recalculated everything because they never bothered filling out that part of the character sheet. Refreshingly, this just becomes an Attack vs Reflex in 4e -- short, simple, and narratively equivalent.
What was Lost?
First, I think that the simplification of attacks vs NADs has lead to a much larger number of such attacks in 4e. In some cases this is personally reasonable -- touch attacks become attacks versus reflex, forced movement often becomes attacks versus fortitude, etc. I do think, however, that there's been a bit of "uniqueness creep" in attack powers that target NADs simply because it's easy or for balance purposes (since such defenses are lower) without a really good justification. It makes it much less special when such attacks happen, and also can tend to devalue investments that players put into their mundane defenses like Armor. Attacks versus Will should be a "Gotcha!" for the usually impervious armor-clad fighter, not an "Oh no not again" moment.
As I mentioned in my post yesterday, rolling dice is fun. Moreover, it becomes more fun the more weighty the effects are, and in general the types of attacks that target non-AC defenses can be some of the most interesting (at least narratively). So while players no longer experience the frustration of having the monster save against their big spell and neutralize their greatness, they also lose the sense of suspense and agency that comes with rolling themselves to see if they resist the Vampire Lord's charm or just barely dodge that fireball's blast in the nick of time. I think to some extent 4th Edition tends to err on the side of focusing on players deriving investment from rolling big attacks and doing awesome things, but I think there is a lot to be said for players reacting to the world around them and taking ownership for what it does to them as well.
Moreover, while moving the attack resolution from defender to attacker streamlines things somewhat when the player is attacking, it can also slow things down if the DM is using something like an area-of-effect attack that now needs to roll individually against all the players. Given that the DM often has enough on his plate to keep him busy, I'm not certain whether this is a net gain for play speed or not. Plus, having players roll their saves during the DM's action gives them something to do and keeps them engaged with what is happening which can mitigate some of the problems with longer combats. I personally think it's a better balance.
Finally, a lot of sustaining effects in 4e rely on players making a succession of "4e Saves" and, as a result, many player abilities that would act to support other players take the form of granting additional save rolls (with bonuses). The save mechanism (especially combined with the condition system) is simple and unified, but I'm not positive that it doesn't lose some expressiveness. If we look back to pre-3e D&D, the idea of having a generic "save" with situational bonuses allows for a lot of more flavorful and specific bonuses than 3e's more generic saves. It's true that 4th edition does allow for bonuses to saves versus attacks with specific damage keywords, but that can be kind of inflexible once you start talking about effects that don't just do damage. Actually, it seems a lot closer to how skills worked in the latest D&D Next Playtest -- a general ability roll with bonuses only if you have a specific skill or trait. Something like that might be a good compromise.
Save mechanics are hard to build both simply and expressively because they are almost by definition a kind of "catch-all" for things that fall outside the domain of basic combat. Since they are more generic, I think that they need to be as derived from basic attributes as possible. I also think that we should pay more attention to what kind of attacks actually require saves -- they should be exceptional and dramatic, not just the same thing with different numbers. Finally, we should consider the real psycho-social effects of whether a player or a DM is rolling the dice, and what the optimal balance around the table really is.